Methodology of fuel cycles long-term safety assessment of SNF/HLW geological disposal

Methodology for the long-term safety assessment of nuclear fuel cycles is given in the presented doctoral thesis. The aim of work was to develop a geological repository model for disposal of spent nuclear fuel (SNF) and high level waste (HLW) using an appropriate software code able to calculate the influence of partitioning and transmutation in advanced fuel cycles. The first step in this process was specifying of indicators which can be used to quantify the radiological impact of each fuel cycle. Indicators such as annual effective dose and radiotoxicity of inventory have been quantitatively analysed to determine the potential risk and radiological consequences associated with production of SNF/HLW. Advanced fuel types bring a number of advantages in comparison to uranium oxide fuel UO2 used worldwide nowadays in terms of safety improvement due to minor actinides transmutation and non-proliferation aspects as well. Within the scope of work, three different fuel cycles are compared from the point of view of long-term safety of deep geological repository. The first considered fuel cycle is the currently used open fuel cycle (UOX) which uses only U-FA (Uranium Fuel Assembly). The second assessed cycle is a closed fuel cycle (MOX) with MOX-FA (Mixed OXides Fuel Assembly) and the third considered one is a partially closed fuel cycle (IMF) with IMC-FA (Inert Matrix Combined Fuel Assembly). Description and input data of advanced fuel cycles have been gained by participation in the EC project RED-IMPACT. Results were calculated using code AMBER, which is a flexible software tool that allows building dynamic compartmental models to represent the migration and fate of contaminants in a system, for example in the surface and sub-surface environment. Contaminants in solid, liquid and gaseous phases can be considered. AMBER gives the user the flexibility to define any number of compartments; any number of contaminants and associated decays; deterministic, probabilistic and time varying parameter values; algebraic expressions to represent the uptake of contaminants by humans, other biota and additional output quantities of interest. This software has been used as well to calculate the total and relative radiotoxicity. The results showed that the advanced fuel cycles MOX and IMF are most perspective from the long-term safety point of view. Calculated maximal annual effective doses are as follows: 8.14x10-9 Sv/year for the cycle UOX, 1.04x10-9 Sv/year for the cycle MOX and 5.69x10-10 Sv/year for the cycle IMF. Maximal annual effective dose occurs at time 6.13x103 years for all considered cycles. Most important contributors to the total dose are long-lived fission products in the period from 103 to 106 years (129I, 14C, 36Cl, 79Se, 99Tc a 135Cs). Later on, the decay series 4n+1 and 4n+2 become dominant, which is related to the slow movement of actinides due to sorption in the host rock. Performed sensitivity analysis showed that Darcy velocity of water movement and distance of the well are ranked among the most important input parameters that affect the values of maximal annual effective dose. Safety indicators have been calculated for amount of waste equivalent to production of 1 TWhe due to comparability of the outcomes. Presented doctoral thesis confirmed possibility to reduce significantly both the radiotoxicity and annual effective dose of advanced fuel cycles MOX and IMF in comparison with the conventional once-through UOX fuel cycle. Works performed and presented results have met expectations and can be finally used for the comparative assessment of the advanced nuclear fuel cycles based on the multi-criteria analysis approach. (Author)